In recent years, there has arisen a question of how to purify water for use in keeping aquarium fishes and living fishes or in transporting living fishes. If the ammonia derived from decomposed excretion of aquarium fishes or feed leftovers is present in water at a concentration exceeding 10 ppm, it would exert a detrimental action on the central nerve system of fishes, resulting in increase of their mortality.
Silicone-coated zeolites have been used as ammonium-ion adsorbents in an attempt to obviate the above problem (Japanese Examined Patent Publication Sho 54-39279). However, such adsorbents act to adsorb about 7% of calcium ions and magnesium ions and are therefore unsuitable for use with seawater as a medium.
Na-type zeolites have been used as ammoniacal nitrogen adsorbents (Japanese Unexamined Patent Publications Sho 53-27387 and Sho 53-131291). These adsorbents are intended to utilize the selectivity of cation ion exchange. Yet due to the selectivity of K.sup.+ .apprxeq.C.sup.2+, Mg.sup.2+ &gt;NH.sub.4.sup.+ &gt;Na.sup.+, the adsorbents used in seawater rich in alkaline earth metal ions are unnegligibly inhibited from adsorbing ammonium ions by calcium ions and magnesium ions.
Conventional adsorbents have various problems to be resolved as described above. Especially there is a strong demand for the development of adsorbents which are capable of selectively and efficiently removing ammonium ions from seawater without likelihood of adversely affecting other seawater components and fishes.
It is an object of the invention to provide an adsorbent capable of selectively adsorbing ammonium ions which is free from the loss due to its dissolution when adsorbing and removing ammonium ions and ammoniacal nitrogen from not only fresh water but seawater, and which is capable of selectively adsorbing only ammonium ions without adsorption of calcium ions, magnesium ions and other mineral components in seawater, is capable of keeping the pH value of seawater varied in an extremely narrow range, and is inexpensive and highly safe, and a process for preparing the same.